Merging Textures into Diffuse
Main concept:

To 'merge' these textures you will need PBR shader as stated by @Craig D Jones in comments section, then setup it properly with simple unwrapped Plane, add some lights (or HDRI), empty texture and finally Bake it.

PBR Shader:

There are numerous free and paid PBR node setups, it's up to you which one you will choose. You can even build one for yourself.

I'm gonna use for this answer the one from Jason Clarke - Simple PBR v5

Note:
This is a valid solution until Blender 2.79 will come out. There will be new shader, Principled BSDF which will do the same for you. I've tested it with newset bleeding edge build.

Object and Material:

Object:

First of all we need some object to bake our materials. Taking that most textures are square shaped we need to add a Plane - Shift+A > Mesh > Plane and check Generate UV's in Tools Panel or hit F6 right after Plane is added to the scene.



Material:

Now go to Node Editor, add Appended (How to append material) Simple PBR Node Group. Connect to it Image Textures accordingly to their names. Remember to set Roughness and Normal to Non-color Data. Also add Normal Map node.



As images I've used free PBR texture set from: http://www.3d-wolf.com/wall.html - Wall 2.

Lights:

This is pretty important step. Final texture look highly depends on it. Here is a little comparison between Albedo, Diffuse (both from texture set), Diffuse RGB (color as a background), Diffuse HDRI (HDRI map as lightning) - both from baking:



As you can see only HDRI map gives pretty neat result. You can also add set of lamps; it's really subjective topic so it depends on you.

How to add HDRI

Note: Go to Top Orthographic View (Num5 > Num7) and enable Rendered Preview (Shift+Z) to see changes, they will be nearly the same as final bake.

Empty texture:

Now when everything is good looking we can prepare texture for baking.

Go to Image Editor, click Image > New Image or Alt+N, set size of new texture to be the same or smaller then original textures, name it as you want (Diffuse in this example).

In Node Editor add another Image Texture node and choose newly created texture. Don't connect it anywhere and keep it selected (important!).



Baking:

Now go to Render tab and scroll to Baking, everything should look like this:



Hit Bake.

When it's done go again to Image Editor, you should have there result of baking. To save it - Image > Save as Image.

Blend file: